Ellipsometry is used to determine the state of polarisation of light. When a polarised light beam interacts with an optical system, the polarisation state of the beam may be modified by the optical system. By careful measurements of the initial and final states of polarisation, properties of the optical system under study may be obtained. This is the general use of ellipsometry. Of special relevance to this invention is the use of ellipsometry for measuring the effective complex refractive index of a small region of a sample.
For the conditions that prevail during the epitaxial growth of crystalline materials by molecular beam epitaxy, the crystal surface is clean and unaffected by oxides and other contaminating layers. Under these conditions it is possible to measure the optical properties of the material without the complicating effects of contaminants. In addition for many materials the refractive index varies with the proportion of constituent components and it is therefore possible to use ellipsometry to measure the composition of the material. This may be very useful in the case of the molecular beam epitaxial growth of compounds. The low energy gap semiconducting material cadmium mercury telluride (Cd.sub.x Hg.sub.1-x Te) is such a material. Here `x` is the molar fraction of cadmium telluride in the alloy.
Cadmium mercury telluride for use in infrared optoelectronic devices is prepared in the form of very thin single crystal layers. The composition required for the layers is determined by the portion of the infrared spectrum in which the devices are intended to operate. In order that device production yields be economically viable it is important that the composition of the layers be controlled to a high level of uniformity across the area over which devices are to be made. A variety of epitaxial growth methods are employed to grow single crystal layers of cadmium mercury telluride. All methods known to us produce thin films with variations in composition larger than 0.001 in the molar fraction of cadmium telluride across a distance of approximately 2 centimeters.